1. Field of the Invention
The present invention relates generally to electric power steering apparatuses for automotive vehicles which provide a steering assist of an electric motor directly to the vehicle steering system to reduce a manual steering effort to be applied by a vehicle driver, and more particularly to an improved electric power steering apparatus which, when its steering torque sensor is out of order or not working properly, can estimate steering torque on the basis of a detected vehicle velocity and steering angle so as to permit a continued controlled operation of the assisting motor on the basis of the estimated steering torque.
2. Description of the Related Art
Electric power steering apparatuses are widely used today which set a target motor current on the basis of a steering torque signal and vehicle velocity signal to drive an electric power assisting motor on the basis of the thus-set target motor current. Specifically, these electric power steering apparatuses feed back a current actually flowing through the motor to minimize an offset between the target motor current and the actual motor current so that feedback control is performed for driving the motor to constantly give an appropriate steering assist to the vehicle steering system. The electric power steering apparatuses control the assisting motor in any of various manners, such as in response to detection of a varying vehicle velocity or number of engine rotations; that is, such motor control schemes are generally arranged to modify output power from the motor on the basis of a detected vehicle velocity and number of engine rotations.
However, because all of these motor control schemes are based primarily on output signals from the steering torque sensor, the electric power steering apparatuses would encounter various inconveniences when the steering torque sensor is out of order or not working properly. Particularly, because of their dependence on the steering torque sensor and vehicle velocity sensor, the electric power steering apparatuses can not properly control the assisting motor when the steering torque sensor is not working.
To avoid the inconveniences, there have been proposed improved electric power steering apparatuses, which use duplicate sets of steering torque sensors, vehicle velocity sensors and CPUs to provide a necessary steering assist from the motor to the steering system in a so-called fail-safe fashion. But, the duplicate provision of the steering torque sensors, vehicle velocity sensors and CPUs would significantly increase the cost of the apparatuses. Besides, in case the main and subsidiary steering sensors both break down, it would no longer be possible to properly control the assisting motor.